The voltage level required to cause a stimulation of the cardiac cavities (ventricular or atrial) is a value typically ranging between 1.5 and 7.5 V, adjustable in step increments of 0.5 V. The amplitude must of course be sufficiently high to cause the depolarization of the myocardium; it is, however, also necessary to avoid too high a value to maximize the lifespan of the power source, because the stimulation energy applied to the myocardium, and thus the corresponding energy consumption of the device, is proportional to the square of the amplitude (and also to the duration) of the pulse delivered.
The test of the effectiveness of the stimulation, i.e., whether the amplitude is sufficient to cause a depolarization, also called a “capture test”, can be carried out at regular intervals, for example every six hours. An automatic test algorithm can be used, such as the algorithm described in the international patent publication WO-A-93/02741, and its counterpart U.S. Pat. No. 5,411,533, commonly assigned to the assignee hereof, Ela Medical. The U.S. Pat. No. 5,411,533 is incorporated herein by reference for the entirety of its disclosure, and in particular the capture test disclosed therein. The capture test essentially identifies a capture threshold amplitude above which a stimulation is effective and below which a stimulation pulse is ineffective. The capture test also may be performed when there is a lost of capture (also known as having a “lost capture”), a condition that occurs when the voltage amplitude required to cause a myocardial contraction either increases spontaneously, or the stimulation voltage applied is decreased by device control, as may occur from time to time. The stimulation pulse amplitude is then adjusted on the basis of the capture threshold thus determined plus a large safety margin. To account for the safety margin, the adjusted level is typically set as twice the measured capture threshold value, between a minimum (typically 1.5 V) and a maximum (typically 5.0 V) limit.
Another technique to which the present invention is referred concerns operating a capture test “cycle to cycle”, i.e., to examine each cardiac cycle to determine whether an applied stimulation was effective. For this purpose, the stimulation voltage is maintained near to the determined threshold voltage at which capture is lost. In the event of a rise in the capture threshold for the patient, the loss of capture (i.e., an ineffective stimulation) is detected by examining cardiac activity during an interval (also called a window), which follows delivered stimulation pulse, typically during the following 63 ms, and determining that there was no myocardial contraction during the window and the stimulation was ineffective. This detection of loss of capture has two consequences. First, at the next stimulation, the stimulation voltage level will be adjusted, i.e., increased by one step, to compensate for the rise in the capture threshold of the patient. Second, and importantly, a backup-stimulation pulse having a higher energy level must be applied immediately (i.e., at the end of the 63 ms window following an ineffective stimulation) to compensate for the absence of a depolarization (spontaneous or stimulated) of the myocardium.
This cycle to cycle technique is particularly advantageous because it permits one to be freed from using the large incremental safety margin in stimulation voltage amplitude applied when the capture threshold was measured only at periodic intervals, for example, every six hours. The lifespan of the power supply based on reduced energy consumption during stimulation can thus be lengthened in a substantial manner. However, the much more frequent need for delivering a backup-stimulation pulse can disturb the operation of pacemaker and can induce an energy consumption that is likely to reduce in the end the benefits from reducing energy consumption obtained by the use of the “cycle to cycle” capture technique. It also is essential in this technique not to delay the backup-stimulation, because such a delay would create a physiological risk for the patient.